1. Field of the Invention
This invention relates to an optical blade drive apparatus, and more particularly to an apparatus which includes a linear motor system which electromagnetically actuates the blade ring for rotation at an extremely high speed.
2. Description of the Prior Art
In general, conventional optical blades for use in an optical instrument such as a photographic camera are mechanically driven so that they can control the amount of light admitted through the lens and prevent light from passing through the lens while the shutter release button is being set. In most cases, the blades of such type have their opening and closing movement associated with other mechanical elements or assemblies. Furthermore, the amount of movement of the blades must be controlled with great accuracy, and mechanisms for driving the blades mechanically, such as power transmission gears and other links and other associated assemblies which control the movement of the blades are complicated and large. As a whole, therefore, the mechanisms are extremely costly.
There is also known a camera of the type which has shutter and diaphragm blades actuated by an electrical control circuit. In all cameras of such type, however, the circuit is simply a timing circuit which controls the timing action of all mechanical elements associated with the blades. Thus, in driving the blades, it is necessary to have energy stored mechanically such as by the cocking of a spring which stores the required energy therein. In the electrically controlled focal-plane shutter, for example, the winding action which sets the shutter first urges the springs of the associated first and second shutter curtains into a cocked state to store the energy therein, and the energy of each spring is then retained by means of a solenoid the action of which is controlled by the electrical circuit. Then, under the control of the same circuit, the solenoids release the springs so as to apply their stored energy to the shutter curtains with proper timing. The shutter drive of such type can be made simpler and smaller in size than the entirely mechanically driven shutter. However, the springs must still be cocked by additional mechanical means, and the energy of the springs must also mechanically be transmitted for driving the shutter curtains. Thus, there is a limit on the reduction in size of the structure which can be attained and in order to make a smaller mechanism, it is necessary to drive all moving parts electrically and directly without an intermediate mechanical structure. For this purpose, there have been proposed a number of methods and structures. In most of these, however, the shutter and diaphragm blades are driven by means of electromagnetic forces provided by a combination of electromagnetic coils, for example, single bobbin coil assemblies, and permanent magnets. Such a driving system is disadvantageous in that the blades are limited in the length of their stroke and in that the coil either must be supplied with a large amount of current or must have an extremely large number of turns so that the blades can be driven at a very high speed. If, however, the number of turns of the coil is increased in a way which does not affect the magnitude of the electrical capacitance therein, the coil becomes of extremely large size. Furthermore, if a coil of a small size is used because of its low capacitance, less current will flow therethrough. Particularly, in a camera whose shutter must be driven at a high speed using a power source and a coil assembly mounted within a camera body of limited space, the disadvantages mentioned above are encountered and become most serious problems.